in exceptional cases(отнюдь не только в математике, как можно подумать, исходя из соответствующей пометы: I will only say that proceedings should be brought before the court by way of appeal in contempt matters only in exceptional casesI. Havkin)
exceptionally(1. used before an adjective or adverb to emphasize how strong or unusual the quality is • The weather, even for January, was exceptionally cold. • I thought Bill played exceptionally well. 2. only in unusual circumstances • Exceptionally, students may be accepted without formal qualifications. OALD Alexander Demidov)